With many electronic devices to which commercial alternating power-supply voltage (AC 100 to 240 V) is supplied, a switching power supply circuit is used for obtaining direct-current voltage for driving an internal electronic circuit. Accordingly, the switching power supply circuit needs a rectifying circuit for converting commercial alternating power-supply voltage to direct-current voltage. If power factor improvement is not made, current flows to a smoothing capacitor connected to the rectifying circuit only at the time of input voltage being near a peak. As a result, high-frequency current and voltage components are generated in the rectifying circuit. These components are a source of high-frequency noise and cause a drop in power factor.
A power factor is a value obtained by dividing input effective power Pi (W), which is the time-average of the product of input voltage and input current in an alternating circuit, by apparent power (which is the product of the effective value of the input voltage and the effective value of the input current). Effective power is obtained by multiplying apparent power by a coefficient (power factor) which depends on a load. If AC 100 V is applied to a simple resistance load, the phases of voltage and current are the same and a power factor is 1. However, the phase of current may deviate from the phase of voltage due to a load other than a resistor. In this case, in order to compensate for a drop in power factor corresponding to the amount of the deviation, it is necessary to increase input current. This causes an increase in power loss on an input line. Therefore, it is necessary to control this power loss by preventing a drop in power factor by the use of a PFC (Power Factor Controller) and to control the above high-frequency noise.
FIG. 13 illustrates a switching power supply circuit including a conventional power factor controller using a fixed on-width control method.
A power factor controller improves a power factor by making the phase of input current equal to the phase of alternating input voltage rectified by a rectifying circuit in a switching power supply circuit, and controls high-frequency current and voltage which cause harmful EMI (Electro Magnetic Interference) or destruction of a device. In the switching power supply circuit illustrated in FIG. 13, alternating input voltage is full-wave rectified by a full-wave rectifier 1. One end of a capacitor 2 is connected to an output side of the full-wave rectifier 1. High-frequency components generated as a result of the switching operation of a switching element 4 described later are removed by the capacitor 2. A step-up circuit including a primary inductor 3 of a transformer T, the switching element 4 which is a MOSFET (Metal-Oxide Semiconductor Field-Effect Transistor), a diode 5, and a capacitor 6 is also connected to the output side of the full-wave rectifier 1. Rectified voltage outputted from the full-wave rectifier 1 is increased and rectified by the step-up circuit. By doing so, a direct-current output voltage of, for example, about 400 volts can be supplied to a load (not illustrated) connected between an output terminal 7 and ground.
A power factor controller 100 includes an integrated circuit into which various kinds of functions are integrated, and has a FB terminal for receiving a feedback signal, an IS terminal for detecting current which flows through the switching element 4, an OUT terminal for output, a ZCD (Zero Current Detection) terminal for receiving a zero cross signal, a RT terminal for connecting a resistor which determines the waveform of the oscillation of an oscillator 13, and a COMP terminal for connecting a phase compensation element. In addition, the integrated circuit includes an error amplifier 11 for amplifying and outputting the difference between the detected value of output voltage inputted to the FB terminal and reference voltage Vref, a PWM (Pulse Width Modulation) comparator 12, the oscillator 13, OR circuits 14a and 14b, a RS flip-flop (FF) 15, a ZCD comparator 16, a timer 17, an OVP (Over Voltage Protection) comparator 18 for protecting against overvoltage, and a comparator 19 for detecting overcurrent.
The RT terminal of the power factor controller 100 is connected to one end of a timing resistor R1 the other end of which is grounded. The ZCD terminal is connected to one end of a secondary inductor 8 of the transformer T via a resistor R2 and the other end of the secondary inductor 8 is grounded. The OUT terminal is connected to a gate terminal of the MOSFET which is the switching element 4. A source terminal of the switching element 4 is connected to one end of a current detection resistor R3 the other end of which is grounded. A point at which the source terminal of the MOSFET and the one end of the current detection resistor R3 are connected is connected to the IS terminal. The output terminal 7 is grounded via voltage division resistors R4 and R5 connected in series. A point at which the voltage division resistors R4 and R5 are connected is connected to the FB terminal. The COMP terminal is grounded via a capacitor C1. A resistor R6 and a capacitor C2 are connected in series and are connected in parallel with the capacitor C1. In addition, the power factor controller 100 has a VCC terminal (not illustrated) for receiving power-supply voltage, a GND terminal (not illustrated) for ground connection.
With the above switching power supply circuit the power factor controller 100 makes the phase of inductor current IL which flows through the step-up circuit equal to the phase of the alternating input voltage full-wave rectified by the full-wave rectifier 1. As a result, its power factor is improved.
The error amplifier 11 of the power factor controller 100 is a transconductance amplifier. The reference voltage Vref is inputted to a non-inverting input terminal of the error amplifier 11 and an inverting input terminal of the error amplifier 11 is connected to the FB terminal. An output terminal of the error amplifier 11 is connected to the COMP terminal and an inverting input terminal of the PWM comparator 12. An output terminal of the PWM comparator 12 is connected to a reset terminal of the RS flip-flop 15 via the OR circuit 14a. The oscillator 13 is connected to the external timing resistor R1 via the RT terminal and generates a sawtooth oscillation output the slope of which corresponds to the resistance value of the timing resistor R1. The oscillation output is supplied to a non-inverting input terminal of the PWM comparator 12. Reference voltage Vzcd is inputted to a non-inverting input terminal of the ZCD comparator 16 and an inverting input terminal of the ZCD comparator 16 is connected to the ZCD terminal.
An output signal from the ZCD comparator 16 and an output signal from the timer 17 are supplied to a set terminal of the RS flip-flop 15 via the OR circuit 14b. An output signal S0 from an output terminal Q of the RS flip-flop 15 is supplied to the gate terminal of the switching element 4 via the OUT terminal. Reference voltage Vovp is inputted to an inverting input terminal of the OVP comparator 18 and a non-inverting input terminal of the OVP comparator 18 is connected to the FB terminal An output terminal of the OVP comparator 18 is connected to the reset terminal of the RS flip-flop 15 via the OR circuit 14a. Reference voltage Vovc is inputted to an inverting input terminal of the comparator 19 and a non-inverting input terminal of the comparator 19 is connected to the IS terminal An output terminal of the comparator 19 is connected to the reset terminal of the RS flip-flop 15 via the OR circuit 14a. 
Operation performed for improving a power factor by the fixed on-width control method is as follows. The ZCD comparator 16 detects a voltage value at which the inductor current IL which flows through the primary inductor 3 of the transformer T included in the step-up circuit becomes zero. When the ZCD comparator 16 detects that the inductor current IL is zero, an output signal from the ZCD comparator 16 becomes H (High) and sets the RS flip-flop 15 via the OR circuit 14b. As a result, the output signal S0 from the RS flip-flop 15 becomes H and this signal is outputted from the OUT terminal. Accordingly, the switching element 4 turns on. In addition, the output signal from the ZCD comparator 16 is inputted to the oscillator 13. When the oscillator 13 is triggered by the output signal from the ZCD comparator 16, the oscillator 13 begins to generate a sawtooth oscillation output (sawtooth signal) at the same timing when the switching element 4 turns on. When the sawtooth signal reaches a determined value, the oscillator 13 stops generating an oscillation output, resets an oscillation output to an initial value, and waits for the next trigger input.
A signal obtained by dividing direct-current voltage outputted to the output terminal 7 by the voltage division resistors R4 and R5 is then fed back to the FB terminal as feedback voltage. An error signal Verr obtained by amplifying the difference between the feedback voltage and the reference voltage Vref is generated by the error amplifier 11. The PWM comparator 12 compares the error signal Verr with the sawtooth signal from the oscillator 13. When the PWM comparator 12 detects that the sawtooth signal has reached the level of the error signal Verr, the PWM comparator 12 outputs a reset signal to the RS flip-flop 15 via the OR circuit 14a. As a result, the output signal S0 from the RS flip-flop 15 becomes L (Low). When the output signal S0 which has become L is outputted from the OUT terminal of the power factor controller 100, the switching element 4 turns off.
If the magnitude of the load connected to the output terminal 7 of the switching power supply circuit is constant at this time, then the error signal Verr is also constant and an on-width of the switching element 4 is time from a point at which the sawtooth signal starts from a reference value to a point at which the sawtooth signal reaches the error signal Verr. Therefore, the on-width is controlled so that it will be constant. However, alternating voltage is inputted to the switching power supply circuit, so voltage across the primary inductor 3 changes according to the phase angle. As a result, the slope of the inductor current IL which flows through the primary inductor 3 of the transformer T changes according to input voltage. The peak value of the inductor current, that is to say, a current value at the timing at which the switching element 4 turns off is proportional to the alternating input voltage and the power factor is improved.
Control methods by power factor controllers are broadly divided into two methods: a continuous current control method and a critical current control method. The above fixed on-width control method belongs to the critical current control method. With the critical current control method, timing at which inductor current IL that flows through an inductor (corresponding to the inductor 3 illustrated in FIG. 13) becomes zero is detected and a switching element is turned on at that timing. The critical current control method detects that the inductor current IL becomes zero, and turns on the switching element 4. Accordingly, soft switching can be realized. Compared with the continuous current control method by which hard switching is realized, turn-on loss is small and efficiency is high. With the critical current control method, on the other hand, the peak value of the inductor current IL is high compared with the continuous current control method. As a result, it is necessary to increase the current capacity of the inductor. Therefore, the critical current control method is used by a power factor controller the power consumption of which is low, for example, about 250 W or less, and is not suitable for a power factor controller the power consumption of which is higher than 250 W.
Therefore, in order to take advantage of the merit of the critical current control method even in a power factor controller the power consumption of which is high, the following control methods are proposed in patent literature 1 through 4. The magnitude of a load is detected by the use of an auxiliary winding (corresponding to the secondary inductor 8 of the transformer T illustrated in FIG. 13). If the magnitude of the load is smaller than or equal to a determined value, then the critical current control method is applied. If the magnitude of the load is greater than or equal to the determined value, then the continuous current control method is applied.
In patent literatures 1 through 3, switching between the critical current control method and the continuous current control method is performed by the use of the fact that time from a point at which a switching element turns off to a point at which inductor current becomes zero becomes longer with an increase in the magnitude of a load (with an increase in the inductor current at the time of the switching element turning off). This is the same with the switching power supply circuit illustrated in FIG. 13. That is to say, in the switching power supply circuit illustrated in FIG. 13, zero current is detected with the secondary inductor 8 of the transformer T as an auxiliary winding. Timing at which the zero current is detected is compared with timing at which determined time elapses on the timer 17 after the turning off of the switching element 4. The RS flip-flop 15 is controlled so that the switching element 4 will be turned on at the earlier timing of the former and the latter. As a result, the continuous current control method is applied in a heavy load region where time which elapses before the detection of the zero current is longer than the determined time specified by the timer 17, and the critical current control method is applied in a light load region where time which elapses before the detection of the zero current is shorter than or equal to the determined time specified by the timer 17.
In patent literature 4, that inductor current becomes zero is not detected directly by an auxiliary winding. The magnitude of a load is determined on the basis of the fact that a positive voltage is outputted from the auxiliary winding at the time of a switching element being in an off state and that a negative voltage is outputted from the auxiliary winding at the time of the switching element being in an on state. That is to say, a capacitor externally connected is charged and discharged by the use of an output from the auxiliary winding so that while the switching element is in the off state, integrated voltage of the capacitor will rise. When the integrated voltage exceeds a determined value, the determination that off time of the switching element is long and that a load is heavy is made and the continuous current control method is applied. The critical current control method is applied in a light load region where the integrated voltage does not exceed the determined value.